I picked up my first L lens last week. I cant stop shooting with it. At night fall I notice the f4. But for $570 how do I complain. Images are super sharp and colors are great. I was first set on getting the 70-200 f2.8 but this lens will hold me out till I set a side an extra 2 grand. The lens is built well and the focus speed is decent. This lens I highly recommend. I heard they are discontinuing it so you might want to get ahold of one fast if that is true. Great Lens got my bang for my buck, for sure !! Tested on my 20d..